1. Field of the Invention
This invention relates to flow field plate designs for reducing the pressure drop in the coolant flow in the coolant transition regions between the ports and the flow fields in solid polymer electrolyte fuel cell stacks.
2. Description of the Related Art
Solid polymer electrolyte or proton exchange membrane fuel cells (PEMFCs) electrochemically convert reactants, namely fuel (such as hydrogen) and oxidant (such as oxygen or air), to generate electric power. PEMFCs generally employ a proton conducting polymer membrane electrolyte between two electrodes, namely a cathode and an anode. A structure comprising a proton conducting polymer membrane sandwiched between two electrodes is known as a membrane electrode assembly (MEA). In a typical fuel cell, flow field plates comprising numerous fluid distribution channels for the reactants are provided on either side of a MEA to distribute fuel and oxidant to the respective electrodes and to remove by-products of the electrochemical reactions taking place within the fuel cell. Water is the primary by-product in a cell operating on hydrogen and air reactants. Because the output voltage of a single cell is of order of 1 V, a plurality of cells is usually stacked together in series for commercial applications. Fuel cell stacks can be further connected in arrays of interconnected stacks in series and/or parallel for use in automotive applications and the like.
Along with water, heat is a significant by-product from the electrochemical reactions taking place within the fuel cell. Means for cooling a fuel cell stack is thus generally required.
Stacks designed to achieve high power density (e.g. automotive stacks) typically circulate liquid coolant throughout the stack in order to remove heat quickly and efficiently. To accomplish this, coolant flow fields comprising numerous coolant channels are also typically incorporated in the flow field plates of the cells in the stacks. The coolant flow fields may be formed on the electrochemically inactive surfaces of the flow field plates and thus can distribute coolant evenly throughout the cells while keeping the coolant reliably separated from the reactants.
To provide both reactants and the coolant to and from the individual cells in the stack, a series of ports are generally provided at opposing ends of the individual cells such that when the cells are stacked together they form manifolds for these fluids. A further design feature required then are passageways to distribute the bulk fluids to and from the various channels in the reactant and coolant flow field channels in the plates. Herein, the regions associated with the coolant are referred to as the coolant transition regions. The coolant transition regions can themselves comprise numerous fluid distribution channels, e.g. oxidant and/or fuel transition channels.
For ease of manufacture and other reasons, a common stack design employs a stack of generally rectangular, planar fuel cells whose flow field plates comprise numerous straight reactant and coolant flow field channels running from one end of the plates to the other. Further, it can be advantageous to employ a stack configuration in which certain of the ports are located on the side of the plates and thus are not in line with the flow field channels. Such a configuration however necessitates directing the associated fluid transverse to the flow field channels in order to fluidly connect the port to the flow field channels in the coolant transition regions. This can be accomplished by forming ducts transverse to any reactant transition channels in these coolant transition regions. As will be more apparent when discussing the Figures below, the presence of such ducts can impede the flow of other fluids in the coolant transition region. Thus, a trade-off can be required between flow through such ducts and flow through other transition channels.
This can be particularly of concern in high power density stack designs that comprise coolant ports located on the sides of the plates and coolant ducts in the coolant transition region that are directed transverse to the flow fields. To achieve the highest power densities, fluid channels are often formed at the limits of reliable manufacturing capability and tolerances. Being a liquid, the coolant flow is subject to greater pressure drops than a gaseous reactant when flowing through ducts or channels of a given size. As a result, the coolant pressure drop can be particularly significant in the coolant transition regions of such high power density stacks, and especially in wider cells where longer transverse coolant ducts must be employed. This can result in non-uniform distribution to and hence non-uniform sharing of the coolant in the coolant flow field channels in the active area of the fuel cell. This in turn increases the risk of overheating (hot spots) and over-drying in the cells on hot days. Also, it can lead to formation of wet spots in the cells, making it difficult to prepare the stack for shutdown in below freezing conditions and also difficult to recover during startup from below freezing conditions. In addition, a high coolant pressure drop necessitates use of a larger, more powerful coolant pump.
The pressure drop can be reduced to some extent by sacrificing space provided for the flow of reactants in the coolant transition region but, depending on port and transition designs, this can result in an unacceptable blocking of the flow of one or both of the reactants. Alternatively, the thickness of the individual fuel cells may be increased and, with it, the height of the coolant ducts in the coolant transition regions. However, the power density of the stack is then undesirably reduced, along with a possible undesirable increase in mass of the stack.
Despite the advances made to date, there remains a need for ever greater power density from fuel cell stacks and more efficient flow field plate designs. This invention fulfills these needs and provides further related advantages.